1. Field of the Invention
The invention relates principally to improving dewatering of a fibrous web by applying steam to the web, typically just as it passes over a vacuum box. In particular, the invention relates to a method and apparatus capable of adjusting the moisture content of a paper web on a paper machine in the cross machine direction to achieve a uniform moisture profile.
2. Review of the Prior Art
It is well known in the art of papermaking to employ steam showers or boxes to improve suction dewatering of pulp and paper webs at various locations on the paper machine. Such showers are especially useful at the "wet end" or forming sections of the paper machine where the web typically exceeds 50 percent moisture content by weight. They are also useful in improving press section dewatering.
A typical steam box impinges dry, saturated or superheated, steam onto the traveling web. The web, supported on a forming wire or drying felt, is simultaneously subjected to a vacuum. The vacuum pulls the steam into the sheet interior where it condenses, giving up its heat of condensation to the water content of the web. The increase in temperature dramatically lowers the viscosity and surface tension of the water content of the web, resulting in a more thorough extraction of water for a given vacuum.
The use of a steam hood generally decreases the overall moisture level of the web, across its entire width. It is the experience of papermakers, however, that significant non-uniformity of moisture content across the width of the web may occur which adversely affects paper machine operation. For example, on a typical machine, the outer edges of the web may be at three percent moisture in comparison with a seven percent moisture content at the center of the web.
It is further the experience of papermakers that various defects in machiney or in its operation in the forming section of a paper machine result in "wet streaks" or areas that have relatively high moisture content with respect to surrounding web areas. Wet streaks may also originate in the press machine section when portions of the press felts become plugged because of faulty felt cleaning shower systems. These streaks appear at unpredictable locations across the width of the web. Wet streaks can run for days before the source of difficulty is found.
The moisture profile uniformity of the web as it leaves the forming section determines to a large extent uniformity in finished paper at the reel. This is so because conventional pressing and can drying are not usually designed to correct local web nonuniformity across the paper machine width.
The speed of the entire machine may be determined by a wet streak, even if only two inches wide. Compensation for one or two regions of wet streaking will often necessitate a reduction in overall water content of the web by several percent in order to build an acceptable reel. These effects on machine speed and steam consumption thus have an important impact on the profitability of the papermaking operation.
The prior art describes a number of schemes for attempting to control the sheet moisture profile in the cross machine direction. Compartmentalized steam hoods, for example, shower a wet streak with extra steam in an effort to reduce overall variability and the potential for rejection at the reel for failure to meet maximum water content specifications.
Dupasquier, in the U.S. Pat. Nos. 3,726,757 and 3,795,578, describes a steam shower divided into 11 compartments across the width of the paper machine, each equipped with a separate steam flow valve. A vacuum box, opposite the shower and under the machine wire, draws steam into the web across the entire width of the machine. Chari et al. "Profile Analysis for Evaluation of a Compartmentalized Steam Box," TAPPI Annual Meeting Preprint (Mar. 15, 1976) describes operation of the Dupasquier hood. The object of the hood is to improve the basis weight and moisture profile by indiviudally controlling steam flow to each compartment. The Chari experiment shows that the profiling steam hood is effective in reducing long-term, cross machine moisture profile variation. The Dupasquier hood resulted in a total reduction of variance of moisture content of approximately 40 percent from a base line value without any significant change in bone dry fiber profiles.
Shelor, U.S. Pat. No. 3,516,607 and Dove, U.S. Pat. No. 3,945,570 also show compartmented steam boxes. In Dove, a portion of the steam is applied across the entire width of the web and another portion of steam is sent through compartmented sections over the web. Shelor is an example of many methods which control the flow of steam into each steam box or compartments by means of a flow control valve.
There are a number of problems with valve-controlled systems. Since typical boxes extend across the width of the web, which may be from 80 to 390 inches, as many as 65 valves may be used. This requires bulky structural systems suspended over the web to support the weight of the heavy valves of the box. Conventional hardware is heat sensitive and thus subject to excessive maintenance problems. Air controlled valves require plastic diaphragms which are so heat sensitive that desuperheated steam must be used. These valves also require pressure regulators, gauges and long control lines, all of which add to the complexity and unreliability of the resulting system.
A principal problem, however, is that the separately valved compartmented boxes lack precision in control. The amount of steam passing through the valve is often unknown and must be determined by means other than merely adjusting the valve control means.
In Wells, U.S. Pat. No. 4,249,992, a method and apparatus was disclosed which avoids a number of valving difficulties by varying the steam discharge opening of each compartment to the web, thus regulating the amount of steam absorbed by the web. These hoods are useful, but require rather complex, unique internal structures which are somewhat bulky and thus unsuitable for use in some mill locations.